Data networks, such as International Business Machine (IBM) Geographically-Dispersed Parallel Sysplex (GDPS) system, can provide storage area networks (SANs) that offer effective disaster recovery for geographically diverse and multi-site enterprises. Low differential latency systems, such as a GDPS application, rely on the relative accuracy of clock time between the various nodes in these multi-site networks. Accordingly, systems or methods are needed to maintain clock accuracy in such applications. For example, In IBM's newest generation of GDPS, timing synchronization is maintained between servers via links capable of supporting IBM's Server Time Protocol (STP). In order for STP to perform within the overall parameters of the GDPS system architecture, the differential latency between transmit and receive directions of a link supporting the STP protocol must be maintained within approximately +/−5 μs.
For example, the optical signals propagating in opposite directions (East-West and West-East) along a point-to-point link in a low differential latency system should have nearly identical transit times, to within approximately 5 μs, which corresponds to the time delay in about 1 km of optical fiber. Because the total transit time includes not only the propagation time through the optical system, but also delays in the electronics that drive the optical signal, the East-West and West-East fiber lengths have to be matched to somewhat better than 1 km. Optical low differential latency systems exist today, but with very short spans (less than 100 km). For example, ADVA Optical Networking (ADVA) has disclosed the use of its Fiber Service Platform (FSP) in conjunction with IBM's STP and GDPS. As disclosed by ADVA, the interoperability of the platform (FSP), protocol (STP), and environment (GDPS) will provide enterprise customers with storage network connectivity of up to 100 km.
There is, however, a desire to extend the reach of low differential latency optical systems beyond 100 km. This typically requires the addition of optical amplifiers and dispersion compensation modules (DCMs), which introduce additional time delays, and potentially additional differential latency, in the system. Although amplifiers typically have low latency in the 0.2-0.4 μs range, conventional DCMs based on dispersion-compensating fiber (DCF) can have up to 70 μs latency, depending on the type and manufacturer. Even for the same DCM type and manufacturer, delay variation from module to module can exceed 10 μs. Therefore, it would be extremely difficult to meet low differential latency system specifications with this type of system.